1. Field of the Invention
The present invention relates to a hand-held drive-in power tool for driving in fastening elements and including a drive-in ram, a guide in which the drive-in ram is displaceably arranged, a drive for driving the drive-in ram and having at least one driving spring member for displacing the drive-in ram, and a device for preloading the driving spring member.
2. Description of the Prior Art
Hand-held drive-in power tools are used for driving fastening elements in constructional components with the drive-in ram. As a drive source for the drive-in ram, a mechanical spring, which is preloaded by a preloading device, is used. The advantage of a driving mechanical spring consists in its cost-effectiveness, so that a drive-in power tool can be economically produced. Further, the advantage of mechanical springs in comparison with the gas springs consists in that preloading of a mechanical spring is not accompanied by increase in temperature as it is the case with a gas spring. Still further, with a preloaded mechanical spring, the stored energy is not lost over a long time, whereas in case of a gas spring, the energy can be completely lost as a result of leakage.
The drawback of mechanical springs in comparison with gas springs consists in that the mechanical springs lose a substantial portion of the energy, which is stored therein, during a rapid expansion because the stored energy is used for acceleration of the spring own mass. Because the mass of a mechanical spring is much greater than that of a gas spring, the corresponding losses in the mechanical springs are much greater in comparison with the gas springs. Because the impact process, which takes place with the drive-in power tools in question, leads to a very rapid expansion of the spring, the above-described phenomenon is very noticeable.
A subject drive-in power tool is disclosed in German patent publication DE 40 13 022 A1. The disclosed drive-in power tool includes an impact mechanism for driving-in nails and which is advanced toward the tool mouth by a spring. An operating device for displacing the impact mechanism in its initial position includes an electric motor and a speed reducing mechanism. The rotary movement of the electric motor is transmitted via the speed reducing mechanism and a cooperating therewith tooth disc to a hammer member of the impact mechanism for displacing the impact mechanism against a spring-biasing force to its initial position in which the impact mechanism is ready for performing an impact process.
The drawback of the known drive-in power tool consists in that the ram speed cannot exceed an amount of 15-20 m/sec which is not sufficient for applications where a high setting energy of about 10-20 J is required, as, e.g., for setting in steel or concrete. This results from the phenomenon, which has already been discussed above, that a mechanical spring must use a portion of energy, which is stored therein, for acceleration of its own mass, which results in the loss of this energy portion for acceleration of the impact mechanism. If order to increase the impact speed of a drive-in power tool, a constructively identical but a stronger spring need be used. Thereby, only own mass of the spring is increased which, again, increases amount of the lost energy, which is used for acceleration of the own mass of the spring, so that, in effect, no increase of speed is achieved.
U.S. Pat. No. 6,708,585 discloses a power tool for driving small nails and having a tubular body in which a ram is displaced. The loop-shaped upper ends of the ram and the body are connected with each other by a rubber band. Before start of a setting process, the ram should be displaced manually relative to the body, tensioning the rubber band. By being released, the ram is displaced in the drive-in direction and drives, with its lower end, a nail in a workpiece.
An object of the invention is a drive-in power tool of the type described above in which the above-mentioned drawbacks of known drive-in power tools are eliminated.
Another object of the invention is a drive-in tool of the type described above in which a high drive-in speed and, simultaneously, a high drive-in energy are achieved with technically simple means.